Well some more current system info would be handy....however
If you plan on upgrading the mobo then u will either need to make sure that the new mobo is compatable with the hardware u already run ie. CPU / PSU / RAM etc. However theres not much point in just upgrading the mobo (unless its broken)

So......

If you get a new mobo you will also need to get a new processer to match the socket type of the mobo
more than likely some new RAM (the more the better and the faster the better)
Unless the PSU is ATX compliant you will need a new one also, however I would suggest getting a nice new psu to go with ur nice new PC. It doesnt really matter about the wattage (450W / 550W etc) its the amparage on the 12v line you need to look for (the more the better)
Optical drives (CD / DVD) you will be able to keep
And a case is up to you (unless the Dell case isnt ATX)

A simple PC setup (like mine at the moment) for the kids to mess around on will probably look like this

Elite K7S5A Pro Skt A
AMD Athlon XP 2400+ Skt A
512MB DDR RAM
120GB Barracuda HDD
nVidia GeForce FX 5200 128MB (or mobo on-board depending on use / budget)
Sound Blaster 64 Sound Card (or mobo on-board as above)
CD ROM / CD RW / DVD ROM / DVD RW (again pick on depending on use / budget / what you already have)
FDD
Cheap (horrible) beige case and thrown in PSU (36A on the 12V)

This cost me 2 years ago approx £400 and if the pc's just for the kids to mess around on, eBay is a great place to pick up cheap parts.

Without really knowing what you alreadyhave or what your going to be using it for I can't really tell you much more, but I hope I might have helped a little anyway